Advanced pressure control systems are widely used in modern automatic transmission control strategies. For example, U.S. Pat. No. 6,807,472, Closed Loop Control Of Shifting Clutch Actuators In An Automatic Speed Change Transmission, describes an embodiment of pressure control solenoid valves, pressure sensors and an electronic transmission controller (ETCU) to perform closed loop pressure control as function of input current commands derived from the sensor outputs. Such systems are complex and require extensive tuning for transient response due to the inherent dynamic variation of the hydraulic system coupled with the inductive loads of the generic solenoids used, generally 35 mH solenoid valve devices. As a result, overall system response is less than favorable due to the limitations of ETCU response and accuracy as well as generic solenoid dynamic response.
Extremely precise pressure command steps (i.e. 5 kPa step amplitudes) can sometimes be required depending on the transmission control strategy employed. Depending on the current control strategies utilized, these very small current commands can be hidden in the ‘noise’ of the closed loop current control. The ETCU (Microprocessor) used to control the input current signal to the solenoid valves is generally a closed loop constant current control based on the conventional P.I.D. methodology, which uses proportional, integral, and/or derivative coefficients to optimize transient current response. The Table below shows the difficulty of tuning the system response and also the limited tuneability of the three parameters, because rise times, overshoot values, and steady state errors are all affected and are interrelated by the coefficients.
CLRTOSSTS-S ErrorKpDecreaseIncreaseSmallDecreaseChangeKiDecreaseIncreaseIncreaseEliminateKdSmall ChangeDecreaseDecrease(SmallChange)where CL is CL response is controlled response, RT is rise time of electrical currrent signal, OS is overshoot of the current signal, ST is settling time of current signal, and S-S error is steady state current error with respect to command current.
This situation is even further complicated when an extremely fast-responding pressure control solenoid valve is installed in the hydraulic system. The extremely low inductance (approximately 6 mH) of a fast-responding pressure pressure control solenoid valve that uses a permanent magnet and solenoid coil motor (e.g. of the type described in U.S. Pat. No. 4,988,074) can easily respond to the regulation profile of the standard closed loop current control. As a result, the improved response of the high speed solenoid valve actually yields less favorable performance (e.g. instability and commanded pressure error) due to the limitations of the ETCU current control.